1. Field of the Invention
The invention relates to a technology of estimating the state of charge of a secondary battery, or the like, and particularly relates to a technology of estimating the full charge capacity from the state of charge.
2. Description of Related Art
It is known that a secondary battery deteriorates due to chronological changes, resulting in reduction of its full charge capacity. If the full charge capacity of the secondary battery is reduced, the amount of electric power that can be used is reduced, and the travel distance of the vehicle (in an EV traveling mode, for example) using electric power is reduced. Therefore, if the full charge capacity of the secondary battery that is in use cannot be grasped with high accuracy, the travel distance of the vehicle using electric power may become shorter than the travel distance corresponding to reduction of the full charge capacity due to chronological changes.
The SOC (State of Charge) of the secondary battery represents the proportion of the current charging capacity to the full charge capacity. Although charge and discharge of the secondary battery can be controlled based on the SOC, the SOC changes if the full charge capacity changes. Thus, if the full charge capacity of the secondary battery cannot grasped with high accuracy, excessive charge and discharge control may be performed on the amount of electric power that can be used.
As a method of estimating the full charge capacity of the secondary battery, the full charge capacity can be calculated, using an integrated value of electric current during external charging, and the SOC estimated at the start of charging and the SOC estimated at the end of charging, for example.
The SOC can be specified from the open-circuit voltage (OCV) of the secondary battery. Since the SOC and the OCV are in correspondence relationship, the SOC can be specified from the OCV if the correspondence relationship is obtained in advance. However, the SOC at each point in time of the start of charging and the end of charging, which is used for estimating the full charge capacity, is grasped from instantaneous conditions of the secondary battery at the specified point in time. Thus, the SOC may not be calculated with high accuracy.
Namely, the OCV is not grasped from conditions of the secondary battery while both of a charging operation and a discharging operation are continuously performed over a given period of time, in other words, from variations in current values and voltage values within a given period, but is simply grasped from conditions of the secondary battery at the start and the end of a charging/discharging operation, or from instantaneous conditions of the secondary battery at certain points in time during a charging/discharging operation. Thus, since the SOC is calculated based on the OCV that is influenced by the usage history of the secondary battery up to each point in time, the full charge capacity may not be calculated with high accuracy. Documents that refer to technologies concerning the charging rate and full charge capacity of the secondary battery include, for example, Japanese Patent Application Publications No. 07-99065 (JP 07-99065 A), No. 2010-217079 (JP 2010-217079 A), No. 2006-284431 (JP 2006-284431 A), No. 2012-063244 (JP 2012-063244 A), No. 2006-292492 (JP 2006-292492 A), No. 2011-106953 (JP 2011-106953 A), and No. 11-38105 (JP 11-38105 A).